Add support for image attributes on remote images

Perhaps also support for width and height although this is not yet supported from remote_stream_wrapper_widget UI afact.

Example Template output after

0 Drupal\Core\Template\Attribute {#4438 ▼
  #storage: array:6 [▼
    "loading" => 
Drupal\Core\Template
\
AttributeString {#4429 ▶}
    "src" => 
Drupal\Core\Template
\
AttributeString {#4437 ▼
      #value: "https://live.staticflickr.com/65535/53494473182_1b00f5a939_b.jpg"
      #name: "src"
    }
    "width" => 
Drupal\Core\Template
\
AttributeString {#4677 ▼
      #value: "1024"
      #name: "width"
    }
    "height" => 
Drupal\Core\Template
\
AttributeString {#3261 ▼
      #value: "576"
      #name: "height"
    }
    "alt" => 
Drupal\Core\Template
\
AttributeString {#4678 ▼
      #value: "Tooting Bec Road, SW16"
      #name: "alt"
    }
    "title" => 
Drupal\Core\Template
\
AttributeString {#4439 ▼
      #value: "Tooting Bec Road, SW16"
      #name: "title"
    }
  ]
}